Noble Metal Surfaces Research Articles

Selective Oxidation of Glucose Over Carbon-supported Pd and Pt Catalysts

Selective oxidation of glucose into gluconic acid by molecular oxygen over carbon-supported Pt and Pd catalysts was studied. Under examination were kinetic regularities of the process in respect of the electronic state of the noble metal surface, dispersion of the active component particles as well as substrate:Pt(Pd) ratio. Catalytic activity of the Pt/C catalysts being normalized to the dispersion of the platinum particles appeared independent of the particles mean diameter in the 1–5 nm range. A negative particle size effect for the Pd/C catalysts, caused by feasibility of oxidation of the surface of noble metal particles with the size less than 3 nm, was observed. Pt/C catalysts exhibited lower specific activity and provided poor selectivity of the glucose oxidation in comparison with Pd/C. Deactivation of Pd/C catalysts arising from the formation of surface Pd(II) oxides was retarded when the reaction was carried out under an oxygen-diffusion control. Selective oxidation of glucose to gluconic acid over carbon supported palladium and platinum catalysts proceeds with the selectivity up to 97 and 77%, respectively. Catalytic activity of the carbon-supported Pt nanoparticles with diameters ranging from 1 to 5 nm towards the selective oxidation of glucose is directly proportional to platinum surface area. Finely dispersed Pd/C catalysts (〈d Pd〉 =3 nm) are prone to deactivation due to oxidation of their surface, while larger metal particles (〈d Pd〉 =6 nm) are more tolerant and stable. The activity of Pd nanoparticles can be maintained when the process is controlled by diffusion of oxygen towards the active component of the catalyst.

Imaging Pauli Repulsion in Scanning Tunneling Microscopy

A scanning tunneling microscope (STM) has been equipped with a nanoscale force sensor and signal transducer composed of a single D2 molecule that is confined in the STM junction. The uncalibrated sensor is used to obtain ultrahigh geometric image resolution of a complex organic molecule adsorbed on a noble metal surface. By means of conductance-distance spectroscopy and corresponding density functional calculations the mechanism of the sensor and transducer is identified. It probes the short-range Pauli repulsion and converts this signal into variations of the junction conductance.

Phonon-induced scattering of excited electrons and holes on (110) noble metal surfaces

A detailed theoretical study of electron-phonon interaction in occupied and unoccupied Ysurface states on Cu110 ,A g110, and Pd110 is carried out. We show that in the unoccupied surface state on Cu110 and Ag110 the e-ph coupling parameter is about three times smaller than that in the occupied surface state due to distinct phonon modes that contribute to e-ph scattering. The difference in mechanisms of phonon-mediated scattering for excited holes and electrons results from distinct spatial localization of occupied and unoccupied states. In spite of both Ysurface states on Pd110 are unoccupied they have spatial distribution similar to that on other considered noble metal surfaces that leads to different phonon-mediated scattering in these states too.

Direct, Electrocatalytic Oxygen Reduction by Laccase on Anthracene-2-methanethiol-Modified Gold

Laccase, a multicopper oxidase, catalyses the four electron reduction of oxygen to water. Upon adsorption to an electrode surface, laccase is known to reduce oxygen at overpotentials lower than the best noble metal electrocatalysts usually employed. While the electrocatalytic activity of laccase is well established on carbon electrodes, laccase does not typically adsorb to better defined noble metal surfaces in an orientation that allows for efficient electrocatalysis. In this work, we utilized anthracene-2-methanethiol (AMT) to modify the surface of Au electrodes and examined the electrocatalytic activity of adsorbed laccase. AMT facilitated the adsorption of laccase, and the onset of electrocatalytic oxygen reduction was observed as high as 1.13 V(RHE). We observed linear Tafel behavior with a 144 mV/dec slope, consistent with an outer sphere single electron transfer from the electrode to a Cu site in the enzyme as the rate determining step of the oxygen reduction mechanism.

Corrosion of Uranium Dioxide Containing Simulated Fission Products in Dilute Hydrogen Peroxide and Dissolved Hydrogen

A combination of electrochemical methods and X-ray photoelectron spectroscopy has been used to study the combined influence of and dissolved on the oxidation of SIMFUEL ( electrodes fabricated to simulate spent nuclear fuel without the accompanying radiation fields) electrodes in 0.1 mol/L KCl (pH 9.5) at . The SIMFUEL electrodes contain (where RE is rare earth) ions at lattice sites within the matrix and noble metal particles dispersed throughout the solid. Under Ar-purged conditions, can be oxidized by by coupling to its direct reduction on the surface and on the noble metal particles that are galvanically coupled to the matrix. The radicals formed on the noble metal particles can be scavenged by reaction with in -purged solutions. Because the concentration is many orders of magnitude greater than concentrations, the rate of formation of radicals on the noble metal surface exceeds that of radicals, leading to the prevention, or even the reversal, of oxidation. A ratio is sufficient to completely protect the surface from oxidation.

Adsorption of small aromatic molecules on the (111) surfaces of noble metals: A density functional theory study with semiempirical corrections for dispersion effects

The adsorption of benzene, thiophene, and pyridine on the (111) surface of gold and copper have been studied using density functional theory (DFT). Adsorption geometries and energies as well as the nature of bonding have been analyzed and compared to experimental results. Dispersion effects between neighboring molecules and between molecules and the surface have been taken into account via a semiempirical C(6)R(-6) approach. The C(6) coefficients for metal atoms have been deduced using both atomic properties and a hybrid QM:QM approach. Whereas the pure DFT calculations underestimate the adsorption energies significantly, a good agreement with experimental results is obtained using the DFT-D method based on the QM:QM hybrid approach.

Atomic engineering of oxide nanostructure superlattices

Low-dimensional nanoscale oxide-metal hybrid structures have been fabricated by decoration of a vicinal noble metal surface, Pd(1,1,17), by 1-D and 2-D Mn-oxide nanowires and nanostripes. STM and LEED demonstrate that highly ordered superlattices of Mn-oxide nanostructures can be obtained. The coupling of the oxide nanophases to the metal steps and terraces leads to a mesoscopic stabilization of the step–terrace morphology thus creating nearly perfect nanopatterned oxide systems.

Coverage‐Dependent Self‐Assembly of Rubrene Molecules on Noble Metal Surfaces Observed by Scanning Tunneling Microscopy

Coverage-dependent self-assembly of rubrene molecules on different noble metal surfaces, Au(111) and Au(100), Ag(111) and Ag(100), is presented. On Au(111), the homochiral supramolecular assemblies evolve with increasing rubrene coverage from very small structures composed of a few molecules, to honeycomb islets, and to one-dimensional chains of supramolecular pentamers. At higher coverage, the racemic mixture of molecules forms close-packed islands. On Au(100), chains of pentamers and two different types of densely packed islands are formed. On the Ag surfaces, exclusively close-packed islands are created, independently of the rubrene coverage. Moreover, the role of the chiral nature of the molecules in the self-assembly process is discussed, as well as the existence of different molecular conformers depending on the supramolecular assembled phase. The observed differences and similarities reflect the influence of the electronic properties and the geometric structure of the various substrates on molecular self-assembly.

Single-Molecule Chemistry of Metal Phthalocyanine on Noble Metal Surfaces

To develop new functional materials and nanoscale electronics, researchers would like to accurately describe and precisely control the quantum state of a single molecule on a surface. Scanning tunneling microscopy (STM), combined with first-principles simulations, provides a powerful technique for acquiring this level of understanding. Traditionally, metal phthalocyanine (MPc) molecules, composed of a metal atom surrounded by a ligand ring, have been used as dyes and pigments. Recently, MPc molecules have shown great promise as components of light-emitting diodes, field-effect transistors, photovoltaic cells, and single-molecule devices. In this Account, we describe recent research on the characterization and control of adsorption and electronic states of a single MPc molecule on noble metal surfaces. In general, the electronic and magnetic properties of a MPc molecule largely depend on the type of metal ion within the phthalocyanine ligand and the type of surface on which the molecule is adsorbed. However, with the STM technique, we can use on-site molecular "surgery" to manipulate the structure and the properties of the molecule. For example, STM can induce a dehydrogenation reaction of the MPc, which allows us to control the Kondo effect, which describes the spin polarization of the molecule and its interaction with the complex environment. A specially designed STM tip can allow researchers to detect certain molecule-surface hybrid states that are not accessible by other techniques. By matching the local orbital symmetry of the STM tip and the molecule, we can generate the negative differential resistance effect in the formed molecular junction. This orbital symmetry based mechanism is extremely robust and does not critically depend on the geometry of the STM tip. In summary, this simple model system, a MPc molecule absorbed on a noble metal surface, demonstrates the power of STM for quantum characterization and manipulation of single molecules, highlighting the potential of this technique in a variety of applications.

Electronic structure and excitations on clean and nanostructured metal surfaces

We present a brief overview of recent studies and new theoretical results for electron-phonon interaction in the Y surface states on FCC(110) noble metal surfaces as well as in surface and quantum-well states of thin films. We discuss the oscillations of electron-phonon coupling parameter λ and the respective contribution to the lifetime broadening of these states. We analyse the effect of spin-orbit splitting of surface states on an electron-electron contribution to lifetimes of excited electrons (holes). Oscillations of the electron-electron contribution and quadratic dependence of the linewidth on energy is discussed for ultrathin Pb(111) films.

Catechol-Grafted Poly(ethylene glycol) for PEGylation on Versatile Substrates

We report on catechol-grafted poly(ethylene) glycol (PEG-g-catechol) for the preparation of nonfouling surfaces on versatile substrates including adhesion-resistant PTFE. PEG-g-catechol was prepared by the step-growth polymerization of PEO to which dopamine, a mussel-derived adhesive molecule, was conjugated. The immersion of substrates into an aqueous solution of PEG-g-catechol resulted in robust PEGylation on versatile surfaces of noble metals, oxides, and synthetic polymers. Surface PEGylation was unambiguously confirmed by various surface analytical tools such as ellipsometry, goniometry, infrared spectroscopy, and X-ray photoelectron spectroscopy. Contrary to existing PEG derivatives that are difficult-to-modify synthetic polymer surfaces, PEG-g-catechol can be considered to be a new class of PEGs for the facile surface PEGylation of various types of surfaces.

Model pseudopotential for the (1 1 0) surface of fcc noble metals

A two-dimensional pseudopotential for the description of electronic structure of the (1 1 0) surfaces of noble metals is proposed. The electronic structure obtained can be useful for rapid calculations of one-particle and collective excitations on the clean (1 1 0) surfaces.

L-Tyrosine on Ag(111): Universality of the Amino Acid 2D Zwitterionic Bonding Scheme?

We present a combined study of the adsorption and ordering of the l-tyrosine amino acid on the close-packed Ag(111) noble-metal surface in ultrahigh vacuum by means of low-temperature scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. On this substrate the biomolecules self-assemble at temperatures exceeding 320 K into linear structures primarily following specific crystallographic directions and evolve with larger molecular coverage into two-dimensional nanoribbons which are commensurate with the underlying atomic lattice. Our high resolution topographical STM data reveal noncovalent molecular dimerization within the highly ordered one-dimensional nanostructures, which recalls the geometrical pattern already seen in the l-methionine/Ag(111) system and supports a universal bonding scheme for amino acids on smooth and unreactive metal surfaces. The molecules desorb for temperatures above 350 K, indicating a relatively weak interaction between the molecules and the substrate. XPS measurements reveal a zwitterionic adsorption, whereas NEXAFS experiments show a tilted adsorption configuration of the phenol moiety. This enables the interdigitation between aromatic side chains of adjacent molecules via parallel-displaced pi-pi interactions which, together with the hydrogen-bonding capability of the hydroxyl functionality, presumably mediates the emergence of the self-assembled supramolecular nanoribbons.

Control of Colloid Surface Chemistry through Matrix Confinement: Facile Preparation of Stable Antibody Functionalized Silver Nanoparticles

Here we describe a simple yet efficient gel-matrix-assisted preparation method that improves synthetic control over the interface between inorganic nanomaterials and biopolymers and yields stable biofunctionalized silver nanoparticles. Covalent functionalization of the noble metal surface is aided by the confinement of polyethylene glycol acetic acid functionalized silver nanoparticles in thin slabs of a 1% agarose gel. The gel-confined nanoparticles can be transferred between reaction and washing media simply by immersing the gel slab in the solution of interest. The agarose matrix retains nanoparticles but is swiftly penetrated by the antibodies of interest. The antibodies are covalently anchored to the nanoparticles using conventional cross-linking strategies, and the resulting antibody functionalized nanoparticles are recovered from the gel through electroelution. We demonstrate the efficacy of this nanoparticle functionalization approach by labeling specific receptors on cellular surfaces with functionalized silver nanoparticles that are stable under physiological conditions.

Mechanistic Study of Surface Functionalization of Enzyme Lysozyme Synthesized Ag and Au Nanoparticles Using Surface Enhanced Raman Spectroscopy

The fate of bioactivity of biomolecules such as enzymes, proteins, and even drug molecules is greatly affected by the conformational changes in the proximity of the solid surfaces. This interaction is the key to the potential of their further applications as biosensors, in drug delivery, etc. With increasing interest in the biofunctionalization of noble metal nanoparticles for various applications, it is important to precisely investigate the functional groups responsible for binding with the nanoparticle surfaces and probable structural changes in the structure of the biomolecules as both are key factors affecting the bioactivity of these molecules once they are tagged onto the nanoparticle surfaces. However, it is not an easy task to probe these properties, especially for bigger molecules such as enzymes and proteins. Surface-enhanced Raman spectroscopy (SERS) has been used extensively in the detection of biomolecules and study of their conformation on noble metal surfaces since its discovery because of...

Inelastic quasiparticle lifetimes of the Shockley surface state band on Ni(111)

We present a study of the low-energy quasiparticle lifetimes of the Shockley surface state on the Ni(111) surface with scanning tunnelling spectroscopy. By measuring the coherence length of the decaying standing wave pattern at straight step edges electron and hole lifetimes have been determined. The values of the lifetime measured on this ferromagnetic surface show to be considerable smaller than the values obtained from noble metal surfaces. This is explained by differences in the electron density of states at the Fermi energy but has to include substantial spin-flip scattering. Furthermore hole lifetimes appear to be larger than electron lifetimes with the same excitation energy. Although only results for the majority spin component are presented, a spin-dependent selfenergy is expected.

Kondo-effect of substitutional cobalt impurities at copper surfaces

The influence of the coordination on the Kondo temperature of a magnetic impurity at a noble metal surface and the line shape observed in low temperature scanning tunneling spectroscopy (STS) is investigated for single cobalt atoms adsorbed on and embedded in copper surfaces. Surprisingly, the Kondo temperature for substitutional cobalt atoms is almost the same as that of adatoms on the Cu(100) surface. This is in stark contrast to the behaviour observed at the Cu(111) surface. DFT calculations reveal that in the case of Cu(100) the coupling of the spin of the cobalt atom to the conduction band is not substantially increased by the incorporation of the cobalt atom. At the same time the observed line shape differs strongly from what is observed on adatom systems.

Plasmonic nanorod metamaterials for biosensing

Label-free plasmonic biosensors rely either on surface plasmon polaritons or on localized surface plasmons on continuous or nanostructured noble-metal surfaces to detect molecular-binding events. Despite undisputed advantages, including spectral tunability, strong enhancement of the local electric field and much better adaptability to modern nanobiotechnology architectures, localized plasmons demonstrate orders of magnitude lower sensitivity compared with their guided counterparts. Here, we demonstrate an improvement in biosensing technology using a plasmonic metamaterial that is capable of supporting a guided mode in a porous nanorod layer. Benefiting from a substantial overlap between the probing field and the active biological substance incorporated between the nanorods and a strong plasmon-mediated energy confinement inside the layer, this metamaterial provides an enhanced sensitivity to refractive-index variations of the medium between the rods (more than 30,000 nm per refractive-index unit). We demonstrate the feasibility of our approach using a standard streptavidin-biotin affinity model and record considerable improvement in the detection limit of small analytes compared with conventional label-free plasmonic devices.

Functionality of novel black silicon based nanostructured surfaces studied by TOF SIMS

A functionality of the novel black silicon based nanostructured surfaces (BS 2) with different metal surface modifications was tested by time-of-flight secondary ion mass spectrometry (TOF SIMS). Mainly two surface functions were studied: analytical signal enhancement and analyte pre-ionization effect in SIMS due to nanostructure type and the assistance of the noble metal surface coating (Ag or Au) for secondary ion formation. As a testing analyte a Rhodamine 6G was applied. Bi + has been used as SIMS primary ions. It was found out that SIMS signal enhancement of the analyte significantly depends on Ag layer thickness and measured ion mode (negative, positive). The best SIMS signal enhancement was obtained at BS2 surface coated with 400 nm of Ag layer. SIMS fragmentation schemes were developed for a model analyte deposited onto a silver and gold surface. Significant differences in pre-ionization effects can play an important role in the SIMS analysis due to identification and spectra interpretation.

Low-pressure plasma cleaning of Au and PtIr noble metal surfaces

The effect of low-pressure hydrogen and oxygen plasma cleaning of Au and PtIr was investigated by X-ray photoelectron spectroscopy. Hydrocarbon contamination was efficiently removed by hydrogen and oxygen plasma. Hydrogen plasma additionally reduces oxygen compounds, especially metal oxides, while oxygen plasma results in the formation of a surface layer of Au 2O 3 and PtO, respectively. Both noble metal oxides are unstable and decompose with time. The decomposition of metal oxides occurs in parallel with the recontamination of the surface. Metal oxides can be removed completely for Au and partially for PtIr by an additional cleaning with hydrogen plasma. Hydrogen plasma treatment is very promising for noble metal surface cleaning.